Recently, from the viewpoint of effective utilization of energy aimed at global environmental conservation and effective utilization of resources, a nighttime power storage system, a home-use distributed electrical storage system based on photovoltaic power generation technology, and an electrical storage system for an electric vehicle, have attracted attention.
The first requirement for the electrical storage elements used in these electrical storage systems is that energy density is high. As the electrical storage element having a high degree of energy density and capable of meeting other storage requirements, lithium ion batteries have been actively sought.
The second requirement is the capacity for high output. For example, in a combination of a highly efficient engine and an electrical storage system (for example, a hybrid electric vehicle), or in a combination of a fuel cell and an electrical storage system (for example, a fuel-cell electric vehicle), high output discharge characteristics are required from the electrical storage system, in order to achieve sufficient acceleration. As one type of a high output electrical storage element capable of corresponding to such requirements, electric double layer capacitors (hereafter, it may simply be referred to also as “capacitor”) using activated carbon as an electrode, have been developed.
The electric double layer capacitors exhibit high durability (cycle characteristics and high temperature storage characteristics), and output characteristics of about 0.5 to 1 KW/L. These electric double layer capacitors are considered to be the optimum electrical storage element in fields where the above-described high output is required; however, the energy density thereof is only about 1 to 5 Wh/L, and output duration time limit their use in practical applications.
As with lithium ion batteries, research continues toward realizing higher output. For example, a lithium ion battery has been developed that is capable of providing a high output of over 3 kW/L, at a depth of discharge (a value indicating a state of charge in terms of percentage) of 50%; however, a lithium ion battery has been actually designed to suppress energy density equal to or less than 100 Wh/L, even though a lithium ion battery is identically characterized by the highest energy density (higher than 100 Wh/L). Durability thereof (cycle characteristics and high temperature storage characteristics) are inferior to that of the electric double layer capacitors. Therefore, in order to have practical durability, the lithium ion battery is usable only in a depth of discharge that is a narrower range than between 0 to 100%. Therefore, usable capacity in practice is reduced, and further research is being carried out to enhance durability.
Although practical application of the electrical storage element having high output, high energy density and durability, as described above, has been required, the lithium ion battery and the electric double layer capacitors have advantage and disadvantage. Accordingly, as an electrical storage element satisfying these technological requirements, development of a lithium ion capacitor has been active.
The lithium ion capacitor is the electrical storage element (non-aqueous lithium-type electrical storage elements) using a non-aqueous electrolytic solution including a lithium salt as an electrolyte, and is the electrical storage element carrying out charge-discharge by a non-faradaic reaction based on adsorption/desorption of a negative ion similar to that in the electric double layer capacitor, in a positive electrode at a voltage of about 3 V or higher, and by a faradaic reaction based on intercalation/deintercalation of a lithium ion similar to that in the lithium ion battery, in a negative electrode.
As described above, in the electric double layer capacitors that carry out charge-discharge by an non-Faradaic reaction in both of the positive electrode/the negative electrode, input-output characteristics is superior (this means that a large current can be charged and discharged in a short period of time), but energy density is low. On the other hand, in a secondary battery that carries out charge-discharge by the faradaic reaction in both the positive electrode/the negative electrode, energy density is superior, but input-output characteristics are inferior. A lithium ion capacitor is an electrical storage element which can achieve both superior input-output characteristics and high energy density, by carrying out charge-discharge based on the non-faradaic reaction in the positive electrode and based on the faradaic reaction in the negative electrode.
As for the lithium ion capacitor, there has been proposed an electrical storage element using an activated carbon as a positive electrode active material, and using natural graphite or artificial graphite, graphitized meso-phase carbon microsphere, graphitized meso-phase carbon fiber, graphite whisker, graphitized carbon fiber or hard carbon, as a negative electrode active material. As the above-described activated carbon, an activated carbon, which had been proposed in the electric double layer capacitor, has been used in the beginning (for example, refer to the NON-PATENT LITERATURE 1 below).
However, between the positive electrode of the lithium ion capacitor and the positive and negative electrodes of the electric double layer capacitor, there is a difference in that a cation in the non-aqueous electrolytic solution is a lithium ion in the former and a quaternary ammonium ion in the latter. Accordingly, attempts have been to find a material that can be used as the positive electrode active material of the lithium ion capacitor, but not divert the activated carbon for the electric double layer capacitor.
For example, there has been proposed an electrical storage element using a hydrocarbon material in which a hydrogen/carbon ratio (atomic ratio) is 0.05 to 0.5, a BET specific surface area is 300 to 2000 m2/g, and having a pore structure of a mesopore volume by the BJH method is 0.02 to 0.3 ml/g, and a total pore volume by the MP method is 0.3 to 1.0 ml/g, for a positive electrode, and using a material where an optically anisotropic carbon material is activation treated, excluding a graphite, for a negative electrode (refer to the PATENT LITERATURE).
There has been proposed a non-aqueous lithium-type electrical storage element using an activated carbon, satisfying 0.3≤V1≤0.8 and 0.5≤V2≤1.0, provided that mesopore volume derived from a pore having a diameter of 20 Å to 500 Å is V1 (cc/g), and micro-pore volume derived from a pore having a diameter of smaller than 20 Å is V2 (cc/g), and a BET specific surface area of 1500 m2/g to 3000 m2/g, for a positive electrode, and using a carbon material satisfying 0.01≤Vm1≤0.20 and 0.01≤Vm2≤0.40, provided that mesopore volume derived from a pore having a diameter of 20 Å to 500 Å is Vm1 (cc/g), and micro-pore volume derived from a pore having a diameter of smaller than 20 Å is Vm2 (cc/g), for the negative electrode (refer to the PATENT LITERATURE 2).